Nanoarchitectures for Metal–Organic Framework-Derived

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Nanoarchitectures for Metal−Organic Framework-Derived Nanoporous Carbons toward Supercapacitor Applications Rahul R. Salunkhe,† Yusuf Valentino Kaneti,† Jeonghun Kim,‡ Jung Ho Kim,*,‡ and Yusuke Yamauchi*,†,‡ †

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Australian Institute of Innovative Materials (AIIM), University of Wollongong, North Wollongong, New South Wales 2500, Australia CONSPECTUS: The future advances of supercapacitors depend on the development of novel carbon materials with optimized porous structures, high surface area, high conductivity, and high electrochemical stability. Traditionally, nanoporous carbons (NPCs) have been prepared by a variety of methods, such as templated synthesis, carbonization of polymer precursors, physical and chemical activation, etc. Inorganic solid materials such as mesoporous silica and zeolites have been successfully utilized as templates to prepare NPCs. However, the hard-templating methods typically involve several synthetic steps, such as preparation of the original templates, formation of carbon frameworks, and removal of the original templates. Therefore, these methods are not favorable for large-scale production. Metal−organic frameworks (MOFs) with high surface areas and large pore volumes have been studied over the years, and recently, enormous efforts have been made to utilize MOFs for electrochemical applications. However, their low conductivity and poor stability still present major challenges toward their practical applications in supercapacitors. MOFs can be used as precursors for the preparation of NPCs with high porosity. Their parent MOFs can be prepared with endless combinations of organic and inorganic constituents by simple coordination chemistry, and it is possible to control their porous architectures, pore volumes, surface areas, etc. These unique properties of MOF-derived NPCs make them highly attractive for many technological applications. Compared with carbonaceous materials prepared using conventional precursors, MOF-derived carbons have significant advantages in terms of a simple synthesis with inherent diversity affording precise control over porous architectures, pore volumes, and surface areas. In this Account, we will summarize our recent research developments on the preparation of three-dimensional (3-D) MOFderived carbons for supercapacitor applications. This Account will be divided into three main sections: (1) useful background on carbon materials for supercapacitor applications, (2) the importance of MOF-derived carbons, and (3) potential future developments of MOF-derived carbons for supercapacitors. This Account focuses mostly on carbons derived from two types of MOFs, namely, zeolite imidazolate framework-8 (ZIF-8) and ZIF-67. By using examples from our previous works, we will show the uniqueness of these carbons for achieving high performance by control of the chemical reactions/conditions as well proper utilization in asymmetric/symmetric supercapacitor configurations. This Account will promote further developments of MOFderived multifunctional carbon materials with controlled porous architectures for optimization of their electrochemical performance toward supercapacitor applications.

1. INTRODUCTION Electric double-layer capacitors (EDLCs) are electrochemical capacitors where energy storage is achieved by electric doublelayer on the carbon surface.1,2 The direct comparison of supercapacitors with other energy storage devices such as capacitors and batteries is shown in Scheme 1. Many previous reports on EDLCs are aimed at addressing the low energy density of supercapacitors.3−5 Nanoarchitectured porous carbons are very useful for solving this problem. Hitherto, carbonaceous materials with different dimensions, such as carbon onions (0D),6 carbon nanotubes (1-D),7 graphenes (2-D),8 carbidederived carbons (3-D),9 activated carbons (ACs) (3-D),10 and templated carbons (3-D),11 have been extensively studied for © XXXX American Chemical Society

EDLC applications. These carbon materials possess various physicochemical properties in terms of surface area, pore size distribution, thermal stability, conductivity, and surface functionality. 12 Despite such wide variations, the most commonly used carbon materials for industrial applications are still ACs with irregular morphologies, poorly graphitized frameworks, and wide pore size distributions because they can be produced in massive quantities from natural carbon precursors. Received: September 9, 2016

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Scheme 1. Comparison of Supercapacitors with Other Energy Storage Devices in Terms of Different Performance Factors

kinds of MOFs, namely, zeolite imidazolate framework-8 (ZIF8) and ZIF-67 (Scheme 2).

Nanoporous carbons (NPCs) with optimized pore size distributions and high surface areas are promising for electrochemical energy storage applications. Traditionally, NPCs can be prepared by a variety of methods, such as templated synthesis, carbonization of polymer precursors, physical and chemical activation, etc.13 Inorganic solid materials such as mesoporous silica14 and zeolites15 have been successfully utilized as templates to prepare NPCs. However, the hard-templating methods typically involve several synthetic steps, such as preparation of the original templates, formation of carbon frameworks, and removal of the original templates. Therefore, these methods are not favorable for large-scale production. Metal−organic frameworks (MOFs) with high surface areas have attracted great interest as a new class of porous materials because they exhibit greater adsorption capacities than other porous materials. MOFs with different particle shapes, porosities, and surface functionalities can be easily prepared by using limitless combinations of organic and inorganic constituents.16,17 However, the low conductivity and poor stability of MOFs have limited their use in energy storage devices.18 MOFs have been demonstrated as suitable precursors for preparing NPC materials, as first reported by Xu and co-workers in 2008 through the impregnation of a secondary carbon source within pores of MOFs.19 A MOF can work as both a sacrificial template and a secondary carbon precursor. During carbonization in an inert atmosphere, the formation of a porous carbon network and decomposition of the MOF can occur simultaneously.20 As most of the content in MOFs is carbon, NPCs can be achieved by direct carbonization of MOFs without the need of additional precursors. The direct carbonization of MOFs was demonstrated by our group21 and Park and co-workers.22 After these reports, MOF-derived NPCs have emerged as a novel type of carbon materials that combine the well-defined shapes of the parent MOFs with high surface areas and large pore volumes, which make them good candidates for energy storage applications. In this Account, we will demonstrate the recent advances in the fabrication of novel MOF-derived carbons and discuss different strategies and challenges for achieving highperformance next-generation supercapacitors.

Scheme 2. Strategic Concepts for the Preparation of Various NPCs from Nanoarchitectured MOFs toward Supercapacitor Applications

2.1. MOF-Derived Carbons from Commercially Available ZIF-8

In 2012, we introduced a direct carbonization method to form MOF-derived NPCs using ZIF-8 as a starting precursor.23 In the early stage of that study, commercially available ZIF-8 powder (Baseline Z1200, purchased from Sigma-Aldrich) was used as the starting material (Figure 1a(i)). To investigate the effect of the carbonization temperature, various temperatures ranging from 600 to 1000 °C were applied to prepare MOF-derived NPCs. Scanning electron microscopy (SEM) images revealed that the resultant NPCs could retain the morphology of the parent MOFs. The average size of these carbon particles was found to be ∼150 nm. The Raman spectra confirmed the presence of both the D and G bands in all of the carbon samples. Furthermore, the IG/ID ratios were found to be constant (∼1) irrespective of the carbonization temperature. Although the applied temperature was varied, all of the samples exhibited a mixed content of graphitic and disordered carbon. This was due to the absence of metals that could act as catalysts for improving the graphitic

2. STRATEGIES FOR UTILIZATION OF MOF-DERIVED CARBONS In this section, we will describe the various fabrication strategies used for developing novel MOF-derived carbons for supercapacitor applications. These studies are mainly based on two B

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Figure 1. (a) Synthesis of commercially available ZIF-8-derived NPCs and electrochemical properties. (i) Schematic representation of the direct carbonization of ZIF-8 crystals and SEM images of the NPCs treated at 900 °C. (ii) Cyclic voltammograms of NPCs treated at various temperatures (Zx, where x is the applied temperature in °C) at a scan rate of 50 mV s−1. (iii) Specific capacitance of Z-900 at different scan rates. (b) Synthesis of ZIF-67derived NPCs and electrochemical properties. (i−iv) SEM and TEM images. (v) Cyclic voltammograms at different scan rates. (vi) Specific capacitance of ZIF-67-derived NPC and AC. Adapted with permission from refs 4 and 23. Copyright 2014 Wiley-VCH and 2012 Royal Society of Chemistry, respectively.

2.2. MOF-Derived NPCs with Different Particle Sizes

degree of the carbon samples with increasing temperature. Compared with ZIF-8-derived NPCs, NPCs prepared by direct carbonization of cobalt-containing ZIF-67 exhibited largely increased graphitization due to the catalytic effect of cobalt nanoparticles (e.g., the IG/ID ratio increased to ∼2.1) (Figure 1b).24 Compared with ZIF-8-derived NPCs and ACs, ZIF-67derived NPCs showed better capacitance retention due to their higher conductivity, which was contributed by the highly graphitic walls (Figure 1b(v,vi)).4 The selection of the metal elements is therefore critical for optimization of the graphitic degree. The surface area gradually increased with increasing carbonization temperature, with the samples heated at 600 and 1000 °C showing Brunauer−Emmett−Teller (BET) surface areas of 24 and 1110 m2·g−1, respectively. In the light of these results, it is confirmed that ZIF-8 alone is sufficient to prepare high-surface-area carbon materials without adding any additional precursors (e.g., furfuryl alcohol, glucose, and glycerol).25−27 Cyclic voltammetry (CV) studies of ZIF-8-derived NPCs were carried out using standard three-electrode measurements in 0.5 M H2SO4 solution (Figure 1a(ii,iii)). The measurements revealed that the NPC sample heated at 900 °C showed a maximum capacitance value of 214 F·g−1 at a scan rate of 5 mV· s−1. However, the capacitance value was found to decrease drastically to 115 F·g−1 (53% of the initial value) at a higher scan rate of 100 mV·s−1. This phenomenon occurs as a result of the poor uniformity in size and shape and the poor conductivity of commercially available ZIF-8-derived NPCs.

The above initial study showed that precise control over both size and shape of the parent MOF is also necessary for enhancing the electrochemical performance. In our follow-up report, we realized this goal by changing the reaction conditions and by utilizing uniformly sized ZIF-8 particles as starting precursors (Scheme 3).28 To obtain small-sized ZIF-8 crystals (average Scheme 3. Formation of Metal−Organic Frameworks and Their Conversion to NPCsa

a

The MOFs are formed in the solution through a coordination reaction between their organic and inorganic constituents. Calcination in a nitrogen atmosphere followed by HF treatment to remove metal particles forms the MOF-derived NPCs. An actual high-resolution TEM image of ZIF-8-derived NPC is shown as well. The scale bar is 5 nm. The dotted lines in the TEM image show the presence of pores.

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Figure 2. Synthesis of ZIF-8-derived NPCs and electrochemical properties. (a) SEM and (b) TEM images. (c) Specific capacitance at various scan rates. (d) Dependence of the specific capacitance on the mass loading at a scan rate of 20 mV s−1 and the corresponding capacitance retention at a scan rate of 200 mV s−1. Adapted with permission from ref 28. Copyright 2014 Royal Society of Chemistry.

diameter ∼300 nm), Zn(CH3COO)2 and 2-methylimidazole were directly mixed in methanol without the use of poly(vinylpyrrolidone) (PVP). To obtain large-sized ZIF-8 particles (average diameter ∼1 μm), Zn(CH3COO)2 and PVP were dissolved in methanol. Then, 2-methylimidazole was separately dissolved in methanol. Finally, these two solutions were mixed together with the samples heated at 900. As shown in Figure 2a,b, the obtained NPCs had well-defined polyhedral shapes with very good uniformity and contained both carbon (∼85%) and nitrogen (∼15%). The NPC synthesized using the size- and shape-uniform ZIF-8 crystals showed a considerably higher specific surface area (1523 m2·g−1) than that prepared from commercially available ZIF-8 (∼1110 m2·g−1). The electrochemical properties of these NPC samples were analyzed using standard three-electrode and two-electrode measurements.28 As expected, the small-sized particles led to serious aggregation, leading to a low capacitance value of 125 F· g−1 even at a low scan rate of 5 mV·s−1. On the other side, as a result of the well-defined 3-D polyhedron shape, the capacitance value of the NPCs derived from large-sized ZIF-8 particles was significantly higher (252 F·g−1) at a scan rate of 5 mV·s−1 (Figure 2c). Furthermore, this material showed a respectable capacitance value of 125 F·g−1 (with a capacitance retention of ∼50%) even at a very high scan rate of 1000 mV·s−1 (200 times higher than the initial scan rate). This high capacitance retention value highlights the advantage of MOF-derived polyhedral carbons with open porous networks, which allow high insertion and deinsertion rates of ions.

The most important parameters in carbon electrochemistry are the variation in capacitance performance as a function of the mass loading on the electrode surface (Figure 2d). It has been found that the capacitance retention value increased accordingly with an increase in mass loading up to high scan rates of 200 mV· s−1. The values are 63%, 74%, and 75% for mass loadings of 1.0, 1.5, and 2 mg, respectively. The symmetric cell constructed from these carbon materials showed a high specific energy of 10.86 W· h·kg−1, which is comparable to that assembled from other carbons reported previously, such as for mesoporous carbon (8.42 W·h·kg−1)29 and mesoporous carbons (9.6 W·h·kg−1).30 Furthermore, this performance is comparable to that of highvoltage-window seaweed carbon (12.6 W·h·kg−1 at a current density of 0.2 A·g−1).31 This NPC exhibited stable performance for up to 2000 cycles with a negligible loss of 8%. These results highlight the benefits of 3-D porous architectures for providing good rate capability, especially at higher scan rates (up to 1000 mV·s−1). 2.3. Core−Shell MOF-Derived Carbons and Hybrid MOF-Derived Carbons

When ZIF-8, which is made from zinc ions coordinated by four imidazole rings, is used as a precursor, the derived carbons are composed of amorphous carbon frameworks with high surface areas and high nitrogen contents. On the contrary, when ZIF-67, which is made from cobalt ions coordinated by four imidazole rings, is used as a precursor, the obtained carbons are composed of highly graphitized carbon frameworks with good electrical D

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Scheme 4. Synthetic Scheme for Hybrid Carbons Using ZIF-8 and ZIF-67; The Hybrid Material Combines Advantages of Both ZIF-8- and ZIF-67-Derived NPCs To Improve the Total Supercapacitor Performance

Figure 3. (a) Schematic illustration of bimetallic ZIFs (Cox·Zn1−x(MeIm)2). (b) Photograph of ZIF-8, ZIF-67, and the bimetallic ZIF (Cox· Zn1−x(MeIm)2) crystals. The Co2+/Zn2+ molar ratios are shown above the bottles. (c) TEM image and (d) Co and Zn elemental maps of bimetallic ZIFs. (e) SEM image of Co0.1·Zn0.9(MeIm)2. Adapted with permission from ref 33. Copyright 2016 Nature Publishing Group.

with a Co2+/Zn2+ molar ratio of 0.05 was found to be 270 F·g−1 at a current density of 2 A·g−1. In comparison, the specific capacitance values of ZIF-8- and ZIF-67-derived carbons were found to be 239 and 119 F·g−1, respectively, at the same current density. These results suggest that the use of core−shellstructured MOFs enables the design of novel NPCs with superior properties compared with those directly derived from ZIF-8 and ZIF-67. This approach clearly demonstrates a new path for improving the electrochemical performance of MOFderived carbons by combining the advantages of two different MOF precursors. The effects of the use of bimetallic MOFs based on ZIF-67 and ZIF-8 on their textural properties have been also studied (Figure 3).33 ZIF-8 and ZIF-67 are miscible because of their similar crystalline structures and lattice parameters. Intrigued by how the different roles of zinc and cobalt ions contribute to the formation of NPCs, various NPCs were prepared from bimetallic MOFs with different Co2+/Zn2+ molar ratios. Starting with ZIF-8derived NPCs, the surface area was found to decrease with increasing Co2+/Zn2+ molar ratio because of the decrease in micropores; however, the content of graphitic sp2-bonded

conductivities, but they tend to have lower surface areas and lower nitrogen contents. As mentioned above, cobalt as an inorganic constituent existing in ZIF-67 can catalyze the graphitization of the carbon component in ZIF-67 during carbonization. Thus, both ZIF-derived carbons have their own advantages and disadvantages. Therefore, the use of novel core− shell-structured MOF precursors may offer a promising way of optimizing the properties of MOF-derived NPCs (Scheme 4).32 Core−shell-structured MOFs were synthesized by using ZIF-8 crystals as seeds for the growth of ZIF-67 shells. Interestingly, after carbonization, the core consisted of NPC with a high surface area and high nitrogen content (∼16 at.%), while the outer shell layer consisted of highly graphitic NPC. The shell thickness was controlled to achieve the highest synergetic cooperation toward the supercapacitor application.32 The BET surface areas of the resulting NPCs gradually decreased from 1499 to 1276, 813, and 496 m2·g−1 when the Co2+/Zn2+ molar ratio was increased from 0.00 to 0.05, 0.35, and 1.00, respectively. When these samples were analyzed by electrochemical analysis using CV with three electrodes, the maximum capacitance value obtained for the core−shell carbon E

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Figure 4. (a) Scheme for the synthesis of the NPC−PANI composite. (b−d) SEM and TEM images of the NPC-PANI composite. (e) Specific capacitance at various scan rates for the NPC−PANI composite, PANI, and NPC. (f) Ragone plot for symmetric supercapacitors based on NPC//NPC (C−C), PANI//PANI (P−P), and NPC-PANI//NPC-PANI (CP−CP). (g) Long-term cycling performance for the NPC−PANI composite. Adapted with permission from ref 44. Copyright 2016 Royal Society of Chemistry.

specific surface area in order to achieve a large capacitance and a high energy density. However, the presence of mesopores in NPCs also plays an important role, as they provide pathways for rapid ion diffusion, which leads to higher specific power.34 Interestingly, MOF-derived NPCs often possess hierarchical micro/mesoporous structures, and it is found that the micropore/mesopore ratio can be controlled by applying different carbonization temperatures.38 These studies revealed that hierarchical meso- and microporous architectures are critical for achieving the desired performance, especially at high current densities.38 Recently, Xu and co-workers39 demonstrated successful control of the conversion of 1-D carbon nanorods obtained from MOF-74 into 2-D graphene nanoribbons. These carbon

carbon was increased, which provided improvements in both conductivity and stability. This study provides a unique example of tailoring the properties of MOF-derived carbon materials by utilizing bimetallic MOFs. 2.4. Control of Other Parameters

Optimized pore structure with high surface area is the key to achieve high performance for supercapacitors. Previous reports have demonstrated that narrowing of the pore size distribution can lead to high energy density in both nonaqueous and aqueous electrolytes.34,35 Furthermore, many theoretical and experimental works have also shown that matching of the pore and ion sizes is very important for obtaining high EDLC performance.36,37 There is another idea slightly different from this. The existence of abundant micropores is indeed beneficial for increasing the F

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Figure 5. (a) “Two-for-one” concept for the preparation of NPCs and metal oxides. (b) Schematic illustration of an asymmetric supercapacitor cell containing nanoporous Co3O4 and NPC as the positive and negative electrodes, respectively. (c) Specific capacitances of NPC and nanoporous Co3O4 electrodes at various scan rates. (d) Ragone plots of symmetric and asymmetric supercapacitors based on NPC and nanoporous Co3O4 electrodes. (e) Comparison of Co3O4//NPC with some reports in the literature (①, ref 18; ②, ref 18; ③, ref 18; ④, ref 52; ⑤, ref 53; ⑥, ref 54; ⑦, ref 55). Adapted from ref 18. Copyright 2015 American Chemical Society.

materials show a high specific surface area of ∼1500 m2·g−1. This

2.5. Hybridization of MOF-Derived Carbons with Pseudocapacitive Materials

work highlights the importance of proper selection of MOF

Even if 1-D and 2-D carbonaceous materials exhibit fascinating electrical properties,41,42 the serious aggregation problem of these materials presents a major concern for commercialization of these materials.43 Recent studies have indicated that 3-D carbon architectures combined with ordered pseudocapacitive materials exhibiting short diffusion paths are excellent for balancing the active surface area and conductivity to improve the specific power without compensating one another.

precursors, which can be further modified to allow better control of the morphology and dimension (possibly 1-D, 2-D, or 3-D) of the resulting NPCs. This result combined with our results demonstrates that the electrochemical properties of NPCs can possibly be improved by achieving more refined control over particle shapes and pore sizes using various types of MOFs.40 G

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Figure 6. Actual device structure using an organic electrolyte for electrochemical measurements. (a) Photograph of the actual electrode used for the cell. (b) Cross-sectional SEM image of the NPC electrode (scale bar is 10 μm in length). (c) Photograph of the HS test cell. (d) Ragone plots of power density vs energy density compared with other previous papers (our data, ref 56; (1), ref 6; (2), ref 6; (3), ref 6; (4), ref 6; (5), ref 42; (6), ref 42; (7), ref 42). Adapted with permission from ref 56. Copyright 2016 Royal Society of Chemistry.

symmetric PANI (12 W·h·kg−1) and the bare NPC (10.26 W·h· kg−1). This performance is superior to those of previously reported symmetric supercapacitors based on other carbons (∼10 W·h·kg−1),1 metal oxides (RuO2,46 18.77 W·h·kg−1; Co(OH)2,47 3.96 W·h·kg−1), hybrid materials (MnMoO4, 11 W·h·kg−1),48 and asymmetric supercapacitors (NiMoO4// carbon,49 12.31 W·h·kg−1; Co(OH)2//graphene,50 11.9 W·h· kg−1). Even though very high current density was applied, the hybrid materials showed good retention compared with previous reports on 1-D and 2-D hybrid materials, which exhibited very low retention performance.51 Even after 20 000 cycles, the loss of capacitance was only 14% of the initial value (Figure 4g). This study demonstrates the benefits of developing 3-D core-shell nanostructures for achieving high electrochemical performance, as they can avoid the serious aggregation problem commonly observed in low-dimensional hybrid materials. By further optimizing the amount of the conductive polymer and the use of different conductive polymers, higher electrochemical performance and longer-term stability can be realized in the future.

Hybrid materials based on ZIF-8-derived NPCs with aligned conducting polymer (polyaniline) nanorods can be designed, as shown in Figure 4a−d.44 As is well-known, polyaniline (PANI) itself is a highly conducting polymer for electrochemical applications, but it exhibits relatively poor stability. Although ZIF-8-derived carbon has shown high surface area, its capacitance value is limited because of its EDLC behavior. However, when we look at the hybrid materials, they easily overcome the disadvantages of these materials by synergistic cooperation of the advantages from both of these materials. The length of the PANI rods can be controlled on the carbon surface by changing the reaction time, and the effect of the PANI layer thickness on the capacitance performance has been carefully investigated. As the reaction time was increased, the PANI nanorod height also increased from 10 to 50 nm, and the overall PANI structure became spongy. As the coating time was increased, the XRD peak intensity at 25° increased, which corresponds to π−π interchain distance increase associated with the improved conductivity of these samples.45 The CV studies show a clear improvement in the capacitance of the hybrid materials. The hybrid materials combine the rectangular shape of EDLC along with the redox behavior of PANI to achieve high performance. The maximum capacitance value of the hybrid materials was remarkably high (over 1100 F· g−1) (Figure 4e). The symmetric supercapacitor tests showed good capacitance performance, high retention, and high stability for the hybrid samples in comparison with ZIF-8-derived NPC and PANI (Figure 4f). The specific energy of the hybrid material was found to be 21 W·h·kg−1 which is much higher than those of

3. TOWARD REAL SUPERCAPACITOR APPLICATIONS USING MOF PRECURSORS 3.1. Fabrication of Asymmetric Supercapacitors from MOF Precursors

Considering an asymmetric supercapacitor application, the supercapacitor performance is obtained by combining the H

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comparable power density. More interestingly, the energy density value obtained for this device is much higher than those of previously reported carbon-onion supercapacitor, MXenes, and graphene-based supercapacitor devices, and it exhibited a 2-fold higher power density than a lithium thin-film battery (Figure 6d).6,42 This work has demonstrated the importance of a MOF-derived NPC for achieving sufficiently high energy density and high power density for consumer electronics applications. This unique properties of MOF-derived NPCs make them worthy to kick-start revolutions in microelectronics and portable device applications.

contributions of the positive and negative electrodes as per the equation13 1 1 1 = + Ccell C1 C2

(1)

where Ccell is the total capacitance of the cell and C1 and C2 are the capacitances of the two electrodes used for the cell configuration. MOFs contain different organic and inorganic constituents, allowing flexibility in the product composition if one can optimize the applied thermal conditions (Figure 5a,b).18 On one side, when the ZIF-67 precursor is heated in a nitrogen atmosphere, the final product is NPCs. On the other side, when the ZIF-67 precursor is heated in air, the organic groups are decomposed and the final product is Co3O4. Furthermore, the original polyhedral morphology of the parent ZIF-67 is maintained in both products. As shown in Figure 5a, in the case of ZIF-67-derived NPC, many nanopores were observed on the surface. In comparison, the nanoporous Co3O4 product was composed of randomly aggregated granular nanocrystals with an average size of 15−20 nm. The surface areas of these samples were found to be 350 and 148 m2·g−1 for NPC and nanoporous Co3O4, respectively. The surface area of the present nanoporous Co3O4 is much higher than that of Co3O4 obtained by traditional methods.18 This is an obvious advantage of MOF-derived metal oxides, which retain the original structure of the parent MOF. The potential window ranges of these materials were determined using three-electrode measurements. The maximum capacitance values obtained were 272 and 504 F·g−1 for NPC and Co3O4, respectively (Figure 5c). In order to show the advantages of these materials in asymmetric configurations, three different cells were fabricated: symmetric cells based on NPC//NPC and Co3O4//Co3O4 and an asymmetric cell based on NPC//Co3O4. The maximum potential window of the asymmetric cell can be extended up to 1.6 V. As a result of the asymmetric configuration, the NPC//Co3O4 cell showed a high specific energy of 36 W·h· kg−1 and a specific power of 1600 W·kg−1. The specific energy and specific power obtained by the asymmetric cell are 4−5 times higher than those of the symmetric cells (Figure 5d). Interestingly, this performance is comparable to those of other asymmetric and symmetric supercapacitors (Figure 5e).18,52−55 The long-term stability of the NPC//Co3O4 asymmetric cell was tested for over 2000 cycles, and the cell showed a very low loss of 11%. Thus, this “two-for-one” concept provides a unique path for the development of high-surface-area carbons and metal oxides using single MOF precursors.

4. CONCLUSIONS AND OUTLOOK In this Account, we have summarized different ways to utilize MOF-derived NPCs to achieve high supercapacitor performance for realizing next-generation energy storage devices. This Account combined with other recent literatures shows that MOF-derived NPCs exhibit outstanding properties that are rarely seen in other carbon materials. Through the use of MOFs as precursors, we can not only realize high surface areas and large pore volumes but also control over the micropore/mesopore ratio by controlling the different metal ions in the MOFs. As the original shapes of the MOFs can be preserved even after carbonization, we can easily tune their dimensions ranging from 1-D to 2-D and 3-D. These possibilities provide opportunities for electrochemists to develop MOF-derived products with the desired electrochemical properties for achieving specific power and energy densities required for commercialized applications. The family of newly developed MOF-derived carbons continues to increase, and currently there are more than 20 000 well-known MOFs. Therefore, there is a tremendous scope to design MOF-derived carbons with different structural properties for supercapacitors using organic and ionic liquid electrolytes for practical applications. Furthermore, computational screening of these materials for different electrolyte interactions and porous networks will provide new insight for the further design of these materials. The residual metal content is another important concern associated with the supercapacitor applications of these carbon materials. To achieve better removal of the metal, more intensive studies regarding the optimization of annealing temperatures and post-heating acid treatments are required in the future. The current major limitation of MOFderived products is that the amounts of the final products are not so large compared to ACs for industrial use. We expect that newly modified synthesis methods as well as thin-film-based devices based on these materials will overcome these issues

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3.2. Organic-Electrolyte-Based Supercapacitors Using MOF-Derived NPCs

The introduction of organic electrolytes ensures the extension of the potential window for carbon-based materials. In order to demonstrate the advantages of MOF-derived carbons over traditional carbon materials, the performance of MOF-derived carbon was compared with that of a commercially available AC sample on the device level (device assembly is shown in Figure 6a−c).56 Two types of symmetric cells based on ZIF-8-derived carbon and commercially available AC were fabricated. The use of the electrolyte tetraethylammonium tetrafluoroborate NEt4BF4 enabled us to extend the possible potential window up to 2.4 V. The ZIF-8-derived NPC shows a maximum volumetric energy of ∼6.6 mW·h·cm−3. Compared with the AC-based supercapacitor, this device shows a higher energy density with

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; [email protected]. ORCID

Yusuke Yamauchi: 0000-0001-7854-927X Notes

The authors declare no competing financial interest. Biographies Rahul R. Salunkhe received his Bachelor’s (2003), Master’s (2005) and Ph.D. (2009) degrees from Shivaji University, India. I

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Yusuf Valentino Kaneti received his B.E. and Ph.D. degrees from the University of New South Wales, Australia in 2009 and 2014, respectively. Jeonghun Kim received his B.S. degree (2007) and Ph.D. degree (2012) from Yonsei University in Seoul, Korea. Jung Ho Kim received his Bachelor’s (1998), Master’s (2000), and Ph.D. (2005) degrees from Sungkyunkwan University, Korea. Yusuke Yamauchi received his Bachelor’s (2003), Master’s (2004), and Ph.D. (2007) degrees from Waseda University, Japan.

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DOI: 10.1021/acs.accounts.6b00460 Acc. Chem. Res. XXXX, XXX, XXX−XXX